[Review] Request from 'aduffeck' @ 'SUSE/machinery/refactor_system_description_loading'

Please review the following changes:
  * c08c2a7 Extract JSON handling into a Manifest class
  * 2d52611 Get rid of SystemDescription#from_json method
  * d8a0df4 Split #from_json into three methods (parse, validate, objectify)
  * 8d29bb4 Merge pull request #457 from SUSE/review_141216_improve_analyze_task_spec
  * b6936f2 Merge pull request #454 from SUSE/review_141216_cleanup_system_description
  * 86b715b Merge pull request #459 from SUSE/fix_autoyast_services_issue
  * 2e1562b Check for existence of services in AutoYaST export
  * dfb540e Use SystemDescription factory instead of FakeFS + stubs
  * 4fa9711 Clean up SystemDescription code a bit